You can feel the horror of the Florida school shooting unfold in this text exchange between two sisters

By Michelle Krupa and Gianluca Mezzofiore, CNN

Updated 8:54 AM ET, Fri February 16, 2018

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(CNN)Facing death, 17-year-old Hannah Carbocci wanted her big sister.

A bullet had just pierced the wall of her classroom at Marjory Stoneman Douglas High School in Parkland, Florida. Gunshots -- a lot of them -- had exploded. Someone in her class was hurt. Someone else, somewhere, was yelling.

From under a teacher's desk, Hannah dialed 19-year-old Kaitlin at work.

Nothing.

So, Hannah did what teenagers do. She texted. And Kaitlin pinged back.

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Hannah Carbocci, 17, left, and her sister, Kaitlin Carbocci, 19

Hannah Carbocci would not be among at least 17 people to die in Wednesday's massacre. But for several excruciating minutes, she and her sister thought she might be.

In those fragile moments, they traded 77 short text messages, which they later shared with CNN. The conversation offers an intimate glimpse into the panic, the fear and the helplessness that gripped a community that day.

On campus, students running from a madman thumbed last words of gratitude and love. Their relatives, out living ordinary days, dialed and redialed, texted and retexted, clinging to hope on their own smartphones.

Here is what Hannah, whose her sister calls her Boobles, and Kaitlin said in what they feared might be their very last exchange:

Hannah

kaitlin there is a shooter on campus

i am not joking

call 911 please

send them to douglas

Kaitlin

hannah what

are you serious rn

Hannah

kaitlin i am not joking they just shot through the walls someone in my class is injured